A magnetic recording medium has been heretofore formed by dispersing a magnetic powder such as iron oxide, a magnetic alloy or chromium dioxide in a synthetic resin binder and coating the obtained dispersion on a nonmagnetic support such as a plastic film, paper or metal. When a signal is recorded on the magnetic recording medium, or the signal is reproduced from the magnetic recording medium, the magnetic recording medium comes into sliding contact with various magnetic heads and guide poles in the magnetic recording apparatus. As a result, the sliding contact abrades the magnetic layer. Residue from the abraded magnetic layer is dispersed to the surroundings, or becomes attached and accumulates on the guide poles or magnetic heads in the form of a powder. This phenomenon deteriorates the recording and reading of a signal from the magnetic recording medium, and also causes dropout. Thus, abrasion of the magnetic layer deteriorates the function of the magnetic recording medium.
In recent years, as magnetic recording medium have been used in more varied applications, there has been an increased demand for improved magnetic characteristics. For example, it has been desired to shift the recording wavelength to a shorter range, and to eliminate the occurrence of dropout. The present inventors have extensively studied problems associated with the recording of short wavelength signals, particularly dropout and clogging of the head. As a result, it was found that a nearly white powder is produced from abrasion of most magnetic tapes which have been heretofore been used. The powder is generated in large amount under conditions of high humidity. When such a magnetic recording medium was operated in a magnetic recording/reproducing apparatus, a visually perceivable amount of residue deposit, dropout and clogging of the head were observed.
These problems were pronounced when the recording wavelength was short and the head gap was narrow.
The present inventors have extensively studied these phenomenon. As a result, it was discovered that a magnetic recording medium which was prepared by removing calcium from a ferromagnetic powder and then coating the powder on a non-magnetic support with a binder results in little or no deposit of the white powder even under high humidity. It was thus found that the occurrence of dropout and clogging of the head upon recording of short wavelength signals (which generally leads to extensive problems of dropout and clogging), is substantially eliminated.
JP-A-56-101649 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a magnetic recording medium comprising a magnetic layer comprising a ferromagnetic metal powder and a binder as main components coated on a nonmagnetic support, wherein the amount of components to be eluted in water constituting the ferromagnetic metal powder is in the range of 1,500 ppm or less. In accordance with JP-A-56-101649, the above described magnetic recording medium exhibits excellent magnetic properties, stability and video characteristics. However, it was found that if a calcium ion is present in the ferromagnetic metal powder, dropout, staining and reduced output result at the surface of the magnetic layer, particularly under elevated temperature and high humidity conditions. The present inventors have extensively studied these problems. As a result, the present inventors discovered that an aliphatic acid which had been incorporated into the system as a lubricant reacted with calcium ion to produce the calcium salt of the aliphatic acid, in which the calcium salt caused dropout, staining and reduced output at the surface of the magnetic layer. The present inventors conducted further studies as to how to eliminate calcium salts of an aliphatic acid. As a result, the present inventors discovered that these phenomena are greatly affected not only by the elimination of calcium ion but also-by the stiffness of the magnetic recording medium, to thereby arrive at the present invention.